Heun's Method program code

%%%% I can't figure out why my function 'HeunIter' doesnot work; even cannot try it! >>>>>>>>>>>>>> function HeunIter(x, y, h, ynew)

    es=0.01;
    maxit=20;
    Call Derivs(x, y, dy1dx);
    ye=y+dy1dx*h;
    iter=0;
    while (ea>es or iter<=maxit)
        yeold=ye;
        call Derivs(x+h,ye,dy2dx);
        slope=(dy1dx+dy2dx)/2;
        ye=y+slope*y;
        iter=iter+1;
        ea=abs((ye-yeold)/ye)*100;
    end

    ynew=ye
    x=x+h
end

%%%% Below is when i try to run function.

>> HeunIter(0,4800,1,2) 'HeunIter'은(는) 정의되지 않은 함수 또는 변수입니다. %%%% It means 'HeunIter' is undefined function or variable. (In korean)

Answer 1

[Hojin Jung]

I also have another function above :

function Derivs(x, y, deriv) deriv=4800-0.6*y/(1-0.025*x); end